This invention relates to new and useful curable epoxy resin compositions comprising acid curable epoxy resins and a minor amount of an oxidizing agent which is capable of reacting with sulfur dioxide to form a catalyst for curing said epoxy resin. These curable epoxy resin compositions are useful particularly in preparing formed, shaped, filled bodies which comprise the epoxy resin and inorganic solid particulates. Particularly useful filled bodies of this type include abrasive articles, foundry cores and molds.
Over the last several decades, the so-called "epoxy" resins have gained wide acceptance in various arts. Epoxy resins possess unusually good electrical, thermal and chemical properties and exhibit low shrinkage during cure. Epoxy resins provide good adhesion to a variety of surfaces of materials, and, consequently, they have been particularly effective in coatings, electronic and electrical applications, and as binders and adhesives in a number of applications. Epoxy resins are characterized by the presence therein of an epoxide group, i.e., ##STR1## wherein x is a small whole number. Such resins are available commercially from a wide variety of sources. Commercially available products, depending upon the chemical makeup of the epoxy resin, are characterized by a variety of properties.
Epoxide resins are converted to useful forms through condensation reactions which are initiated and/or promoted by means of heat and/or a condensing catalyst or curing agent. Conventionally, the epoxy resins are marketed in the form of liquid or solid compositions comprising the partially condensed resin which may or may not contain the curing agent. A final cure to a solid usable form is effected at the time of use, if a catalyst is present, by heating the composition to a moderately elevated temperature. In many instances, the commercially available epoxy resin compositions will not contain the curing agent or hardener because many curing agents have a certain degree of activity at ambient temperatures, and consequently such compositions would have an undesirable short storage life. Accordingly, producers of epoxy resins sell hardener packages for use with epoxy resins and, generally, such hardeners or curing agents are added to the epoxy resin when it is desired to cure the resin to a solid usable form.
Another method for effecting the cure or hardening of epoxide resins, particularly at ambient temperatures and without requiring the incorporation of a curing agent in the composition itself have been described. For example, in U.S. Pat. No. 3,139,657, epoxide resin compositions are cured through the use of normally gaseous curing agents. Examples of such curing agents includes certain inorganic nitrogen and halogen compounds such as hydrogen halides, boron-halogen compounds, silica-halogen compounds and nitrogen-halogen compounds. It is essential, however, that the epoxide composition be in contact with a solid heat absorbent during its treatment with the gaseous curing agent and that the epoxide compositions be in the form of a thin film with its surface area exposed for contact with a gaseous agent. The patentees state that films greater than about 0.01 inch thickness cannot be completely cured by this process.
In the foundry industry, sand is coated with resin binders and formed into molds and cores for the production of precision castings. A wide variety of techniques has been developed for the manufacture of sand cores and molds. These involve the hot box technique for mold and core formation; the shell method; the "No-Bake", and the cold-box technique.
In the hot box and shell methods, sand molds and cores are formed by heating a mixture of sand with a thermosetting resin at a temperature of about 150.degree.-320.degree. C. in contact with patterns which produce the desired shape of the mold or core. The resin is polymerized and a core or mold is formed. Procedures of this type are described in Dunn et al U.S. Pat. No. 3,059,297 and Brown et al U.S. Pat. No. 3,020,609.
A particular disadvantage of the hot box and shell methods is the necessity for heating the pattern boxes to 150.degree.-320.degree. C. to polymerize and cure the resin binder. This involves considerable expense and is generally a high cost technique.
The cold box techniques for core and mold formation involve the use of sand mixed or coated with resin which may be cured at room temperature by acid or base catalysis. Acid or base catalysts have been used in liquid, solid or gaseous form. Typical cold box processes are shown in Blaies U.S. Pat. No. 3,008,205; Dunn et al U.S. Pat. No. 3,059,297; Peters et al U.S. Pat. No. 3,108,340; Brown et al U.S. Pat. No. 3,184,814; Robins U.S. Pat. No. 3,639,654; Australian Pat. No. 453,160 and British Pat. No. 1,225,984. Many of these processes involve the use of sulfur-containing acid catalyst such as benzene sulfonic acid, toluene sulfonic acid and the like.
A few years ago, a process was developed for room temperature polymerization of condensation resins in which an acid-curing agent is generated in situ in the resin or on a sand-resin mix. It has previously been suggested in U.S. Pat. No. 3,145,438 to inject SO.sub.3 in a form of a gas into a mixture of sand and resin to cure the resin at room temperature. It was found, however, that this process causes an instantaneous curing of the resin in the region subjected to treatment by SO.sub.3 which impedes the diffusion of this gas to other parts of the resin, particularly the central parts of the mixture.
Subsequently, a method was developed which avoided this difficulty. In Richard U.S. Pat. No. 3,879,339, it is disclosed that sand may be coated with a suitable oxidizing agent, such as an organic peroxide, and coated with the resin to be used in binding the sand into the form of a core or mold. The sand-resin mixture is then formed into suitable shape and treated with gaseous SO.sub.2. The SO.sub.2 is oxidized, in situ, to SO.sub.3 and converted to sulfur-containing acid by water present in the mixture. The sulfur-containing acid which is generated in situ causes a rapid and uniform polymerization of the resin at room temperature. This process has proved successful commercially and has been applied to phenolic resins, furan resins, and urea-formaldehyde resins, as well as mixtures and copolymers thereof.
In the cold box method of Richard U.S. Pat. No. 3,879,339, there are a large variety of peroxides disclosed which may be added to sand along with resins which are used in forming sand cores or molds. This composition is subsequently formed into shape and treated with gaseous SO.sub.2. The peroxides which are disclosed in the Richard patent are mostly quite expensive and, in many cases, are difficult to handle and to ship or transport. Organic peroxides require special approval for transportation in interstate commerce. Organic peroxides are often highly flammable or present other fire hazards. Organic peroxides also are often shock sensitive and may explode or detonate under certain conditions. As a result, not all organic peroxides can be used in the Richard process because of economic and safety considerations.
The prior art discloses some applications of epoxy resins to the manufacture of foundry cores and molds but these resins usually require the use of strong acid gases for curing which can give rise to serious problems, e.g. corrosion, etc., with the equipment and difficulties in the use of the epoxy bonded products.
Kottke et al U.S. Pat. No. 3,145,438 discloses a gaseous curing process for curing resins for binding sand and abrasive granules, i.e. sand cores and molds. The resins disclosed are primarily furfuryl alcohol formaldehyde resins and also certain epoxy resins including epoxidized linseed oil, mono- and divinylcyclohexene dioxide, butadiene dioxide, glycidyl methacrylate and its polymers. The curing gases are strong acid gases including boron trifluoride, boron trichloride, hydrogen chloride, sulfur trioxide.
Moore U.S. Pat. No. 3,107,403 discloses the preparation of sand cores or molds using epoxy resins in which the molding composition is cured by treatment with a strong Lewis acid, such as boron trifluoride, titanium tetrachloride, tin tetrachloride, etc.
Walker et el U.S. Pat. No. 3,428,110 discloses a process for preparing sand cores and molds using polyisocyanates and a curable resin, e.g. phenolic, which is cured by treatment with a gaseous amine.
Bickerdike et al U.S. Pat. No. 3,519,056 discloses a method of manufacturing a mold for metal casting in which the mold is formed of mineral fibers and includes a thermosetting resin including an epoxy resin or a phenolic resin. The resins are cured at a high temperature.
Stewart et al U.S. Pat. No. 4,176,114 discloses the preparation on foundry cores and molds using resins modified with polyfurfuryl alcohol. The sand is treated with resin and an organic peroxide or hydroperoxide and the gassed with sulfur dioxide.
Steinbacher U.S. Pat. No. 4,050,500 discloses a shell molding process using a soluble silicate, such as sodium silicate, as a binder. The sodium silicate is used to bind the sand in addition to a thermosetting organic resin. The resins are cured by treatment with peroxides such as benzoyl peroxide or t-butyl perbenzoate.
British Pat. No. 2,066,714 describes processes for preparing shaped foundry articles using a binder material which comprises an ethylenically unsaturated monomer and curing by means of a free radical initiator. The initiator comprises an organic peroxide and a catalytic agent such as sulfur dioxide. Examples of monomers described as being useful include acrylates, and modified acrylate copolymers such as epoxy-acrylates, polyether-acrylates, polyester-acrylates and polyester-urethane-acrylates.